Repeated creep events on the San Andreas Fault near Parkfield, California, Recorded by a strainmeter array

Abstract

Following the 1966 Parkfield‐Cholame (California) earthquakes a creepmeter was installed across the fresh surface break of the San Andreas fault at Carr Ranch (now part of the Jack Ranch), 10 km southeast of Parkfield. It has recorded continuing slip which since the end of 1968 has occurred at about 10 mm yr^(−1), occurring primarily in discrete events at intervals of a few months. In April 1976 an array of four strainmeters was installed near this creepmeter at distances between 0.2 and 2.2 km from the fault to detect the elastic strain fields associated with creep events on the fault. Four similar sets of signals have since been observed on the strainmeter array, separated by intervals of 4 to 5 months. A week after three of these events, creep began at the Carr Ranch creepmeter, but no signals were detected by the strainmeters while the creepmeter was recording slip. Analysis of the strainmeter signals shows that they can be modeled by a slip zone on the fault 640 m long and extending from 30 to 510 m in depth, with right lateral slip of about 3.5 mm. Propagation of a dislocation from the lower northwest corner of the slip zone to the upper southeast corner is indicated. The phenomenon may be due to an asperity or an area of higher friction on the fault at Carr Ranch, which is repeatedly loaded to failure by steady slip on the fault around it. This is the first report of a well‐defined fault creep event which repeats itself with a high degree of similarity and which has been observed at a distance of over 2 km from the fault.